So, one of our players/DM is out of town this weekend and I wanted to try and run a one shot since I had time to prep. I'm running it for two players that wanted a high level session (either 15ish or 20). I've only DM'd a few session, so I'm a little intimidated about doing a high level one shot, even thought it will mostly be combat since you can't really create story over one session.
If you don't feel comfortable doing it, you don't have to do it. Slowly growing into GMing is way more rewarding and way less frustrating than rushing for milestones.
Anyway, if you want to stick with the idea, then better keep it simple. I'd go for some arena, let them fight against diverse foes of increasing difficulty. Flavor it like you want (traditional imperial arena, mad wizard's test area, competition to become Gorum's champions etc.) and add some fancy details (e.g. jubilation from sound files, a grumpy healer between fights or different threats / bribes from different enemies). Let them go for some epic prize - after all, it's a oneshot, they can't ruin a campaign with it. And have a backup plan in case they lose early - maybe a loser bracket or something.
If you can choose between 15 and 20, go for 15. That will be taxing enough, also for your players - unless they are quite some experts in high level play. Finally, don't waste your time building NPC casters at such a level - take them from NPC Codex (with little modifications, if you feel like it) or slap class templates from Monster Codex on some monsters.
My recommendation is Academy of Secrets. It's written for level 13, but if your players are set on 15+ you might be able to tweak the encounters a bit (Advanced template on the higher encounters). I have a group who tried playing it in PbP, unfortunately things got in the way and it fell apart, but not before learning that it's a pretty cool romp.
I won't give too much away, but my opinions are spoilered here anyway.
Maybe settle for the midlevel "best one shot in ages" : Up from Darkness.
If you're set on high level read the level 16+ thread first.
From Paizo The Witchwar Legacy is for level 17.
Adventure Quarterly #2 contains a lvl 18 adventure:
The high level module of this installment is Steven D. Russell's "Dungeon of No Return", which could e considered a nod towards the "Tomb Of Horrors". If the name was not ample clue: PCs will DIE here.
AQ feature one high level adventure in almost each issue...
Or start the converted A Paladin in Hell with the early parts as vignettes skipping the middle parts... (it's not a one shot)
Run Amok Games has the Kingdom of Toads.
The high level is mainly coming from the two players, as they aren't new players. They have played 3.5 and have played Pathfinder for 1~2 years now.
My original idea was a bit of a castle. I wanted to borrow something from 4th Edition D&D to see how my players reacted to it (Skill challenges), something Matthew Colville brought up in one of his videos.
https://www.youtube.com/watch?v=GvOeqDpkBm8
I will probably try out Up from Darkness, I'll see how against mid level one shot the players are.
The main issue I have with high level PCs is all the immunities and counters they have.
I would seriously suggest reviewing their finalized character sheets and note all the immunities and counters they have. And plan accordingly.
This doesn't mean never shooting fire against a fire immune PC, it means don't fill the dungeon with a ton of pyromaniacs.